I am on Metformin. Once in a while, my ring finger on my left hand turns red as you describe. It just suddenly appears and takes several days to clear up. I have no idea what causes it. It started about 2 months ago and has happened 3 or 4 times. I don't think Metformin is causing it because I've been on it for about a year. I began to think it might be caused by a new hand cream (stopped that) or some antibacterial wipes that I have been using at work but that doesn't explain it only happening to one finger. I wish I could answer your question. I hope you figure out what is causing it.